Destroy UDP UMA zones (empty or not) upon network stack teardown

to not leak them making the VM subsystem unhappy with every stoped vnet(*).
We will still leak pages (especially as zones are marked NOFREE).

(*) This will also keep vmstat -z more usable.

Sponsored by:	ISPsystem
MFC after:	5 days

sys/netinet/udp_usrreq.c

@@ -245,6 +245,9 @@ udp_destroy(void)
 	    V_udbinfo.ipi_hashmask);
 	hashdestroy(V_udbinfo.ipi_porthashbase, M_PCB,
 	    V_udbinfo.ipi_porthashmask);
+
+	uma_zdestroy(V_udpcb_zone);
+	uma_zdestroy(V_udbinfo.ipi_zone);
 	INP_INFO_LOCK_DESTROY(&V_udbinfo);
 }
 #endif